Method of manufacturing a sealed electrochemical cell

ABSTRACT

An electrochemical cell comprising a negative electrode whose electrochemically active material includes an intermetallic compound which can form a hydride with hydrogen, the cell comprising an electrolyte of a solution of at least one alkali hydroxide in water, the negative electrode being provided with a discharge buffer by introducing a quantity of an alkali metal, alkaline earth metal or aluminium into the cell before the cell is sealed, the quantity corresponding to the desired quantity of hydrogen in the negative electrode in the uncharged condition of the cell.

This is a division of application Ser. No. 348,224, filed May 5, 1989,now U.S. Pat. No. 4,943,500.

The invention relates to a sealed electrochemical cell comprising anegative electrode whose electrochemically active material includes anintermetallic compound which can form a hydride with hydrogen, the cellcomprising an electrolyte of a solution of at least one alkali hydroxidein water, and the negative electrode having a discharge buffer.

The invention also relates to a method of manufacturing such a sealedelectrochemical cell.

A rechargeable cell of this type is described in European PatentApplication No. EP 170519. In this cell the negative electrode has alarger electrochemical capacity than the positive electrode to precludethe formation of gaseous hydrogen in the case of overcharging. In thecase of overdischarging, which may occur, for example, when such cellsare connected in series in a battery, oxygen evolution and corrosion maycause damage to the negative electrode. To preclude this a part of theexcess capacity of the negative electrode is filled with a precharge ordischarge buffer. The purpose of these measures is to increase theoperating life of the electrochemical cell, i.e., to increase the numberof possible charging and discharging cycles. Moreover, overdischargingwould lead to a substantial pressure increase in the cell. If thiscauses a safety valve to open, electrolyte escapes from the cell and,hence, the capacity of the cell decreases.

Because it is difficult to safely precharge cells on a large scale bysupplying hydrogen gas to the cells before they are sealed, aprecharging element is used in the above-mentioned European PatentApplication. This precharging element comprises a base metal M, forexample a transition metal or an element of one of the groups III and IVof the Periodic Table of Elements, and is electrically conductivelyconnected to the negative electrode. This negative electrode is partlycharged by oxidation of the precharging element according to thefollowing reaction:

    M+nOH.sup.- →M(OH).sub.n +ne.sup.-

and by electron transfer to the negative electrode.

This known method has the disadvantage that foreign ions are present inthe electrolyte. Moreover, to provide the cell with a prechargingelement which is connected to the negative electrode the construction ofthe cell must be adapted.

It is an object of the invention to provide an electrochemical cell of asimple construction, in which the negative electrode has a dischargebuffer without an external supply of hydrogen gas. An additional objectof the invention is to provide such a cell, in which the composition ofthe electrolyte is not disturbed by the presence of foreign, inparticular, multivalent ions. A further object of the invention is toprovide a method of manufacturing such an electrochemical cell, whichmethod provides a desired quantity of a discharge buffer in areproducible manner.

According to the invention, this object is achieved using anelectrochemical cell as described in the opening paragraph, which cellis characterized in that the discharge buffer is obtained by thepresence of an alkali metal, an alkaline earth metal or aluminium incontact with the electrolyte.

The composition of the electrolyte is least influenced by using analkali metal, in particular an alkali metal the ion of which is presentin the electrolyte already in the form of the alkali hydroxide.

Shortly before the cell is sealed, it is filled with electrolyte and thefollowing reaction occurs between the alkali metal or alkaline earthmetal M and water from the electrolyte:

    2M+2nH.sub.2 O→2M(OH).sub.n +nH.sub.2

The hydrogen formed is absorbed in the negative electrode which thusobtains a discharge buffer. The construction of the cell does not haveto be changed because electrical contact between the metal M and thenegative electrode is not necessary. The alkali metal or alkaline earthmetal is simply introduced into the cell in the form of a wire, metalshavings or metal grains before the cell is sealed.

In a very suitable embodiments of the cell according to the invention,the discharge buffer is provided by the presence of lithium in contactwith the electrolyte. Using lithium has the advantage that there is novigorous reaction when lithium is brought into contact with theelectrolyte.

According to the invention, the object of providing a method ofmanufacturing an electrochemical cell is achieved in that the dischargebuffer is provided by introducing a quantity of an alkali metal oralkaline earth metal into the cell before the cell is sealed, thequantity corresponding to the desired quantity of hydrogen in thenegative electrode in the uncharged condition of the cell.

The invention will be explained in greater detail with reference to anembodiment and an accompanying drawing, in which the sole FIGURE ispartly a sectional view and partly an elevational view of a sealedrechargeable electrochemical cell according to the invention.

EXAMPLE

The cell which is shown in the FIGURE and which is sealed from the airconsists of a suitable housing 1 of metal, such as stainless steel,provided with a cover 2 having apertures for the conductors 3 and 4. Bymeans of synthetic resin rings 5 the conductors are insulated from themetal housing (1,2). At the outside the housing may have a diameter of,for example, 22 mm and a height of 41 mm. A wound section which iscomposed of a negative electrode 6, a separator 7 and a positiveelectrode 8 is provided within the housing, the assembly being envelopedby an electrically insulating synthetic resin foil 9 of, for example,polyvinyl chloride and bearing on a disc 10 of an electricallyinsulating material such as polyvinyl chloride.

Shortly before the cell is filled with electrolyte and sealed, aquantity of lithium 11, for example, in the form of metal shavings or asmall block is introduced into the cell, the hydrogen gas then formedbeing absorbed by the negative electrode. When the reaction is completedsome time after the cell has been sealed no metallic residues of lithiumremain.

When the alkali metal is used in the form of a wire, it is advantageousto incorporate the wire in the wound section (6,7,8).

The negative electrode 6 comprises a hydride-forming intermetalliccompound and is connected to the conductor 3. The hydride-formingcompound has, for example, the composition La₀.8 Nd₀.2 Ni₂.5 Co₂.4Si₀.1. Other suitable hydride-forming compounds are those described in,for example, U.S. Pat. Nos. 4,487,817, 4,699,856 and 4,702,978, andalso, for example, TiNi alloys. The negative electrode is produced byfusing appropriate quantities of the relevant elements, pulverizing theintermetallic compound thus formed and applying it to a nickelsubstrate, for example with the aid of a paste comprising a polymerbinder material such as polyvinyl alcohol. The paste is, for example,composed of 75% by weight of the intermetallic compound, 24.5% by weightof water and 0.5% by weight of polyvinyl alcohol.

The positive electrode 8 is a nickel hydroxide electrode of theconventional sintered type, and is connected to the conductor 4. A 6Nlithium hydroxide and potassium hydroxide solution (molar ratio 1:1) inwater is used as the electrolyte. The electrolyte is absorbed in theseparator 7 and so contacts the electrochemically active material of thetwo electrodes that it is wetted thereby. The separator is in the formof a non-woven sheet of polyamide fibers.

The free gas space in the cell is approximately 5 cm³. A sealed cell ofthis type has an EMF of between 1.2 and 1.4 V. The cells according tothe invention can be assembled in a conventional manner to formbatteries comprising, for example, a plurality of series-arranged cells.

A cell which is sealed from the atmosphere may comprise a valve of sucha dimension that it becomes operative at a predetermined pressure.

In a rechargeable cell of the sealed type the electrochemically activeportion of the positive electrode is composed of, for example, nickelhydroxide, silver oxide or manganese oxide, nickel hydroxide generallybeing preferred for practical reasons.

The electrolyte used in the cell generally consists of a solution inwater of one or more alkali hydroxides, such as lithium hydroxide,sodium hydroxide and potassium hydroxide, having a pH exceeding 7.

In addition, the cell may comprise a separator which electricallyinsulates the electrodes, but permits ion and gas transport Theseparator may consist of (woven or unwoven) synthetic resin fibers, forexample polyamide fibers or polypropylene fibers, and is preferablyhydrophilic.

We claim:
 1. A method of manufacturing a sealed electrochemical cell comprising a negative electrode whose electrochemically active material includes an intermetallic compound which can form a hydride with hydrogen, the cell comprising an electrolyte of a solution of at least one alkali hydroxide in water, and the negative electrode having a discharge buffer, characterized in that the discharge buffer is provided by introducing a quantity of an alkali metal, alkaline earth metal or aluminium into the cell before the cell is sealed, the quantity corresponding to the desired quantity of hydrogen absorbed in the negative electrode in the uncharged condition of the cell.
 2. A method as claimed in claim 1, characterized in that lithium is used to provide the discharge buffer. 